1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a facsimile device and a printer, which sends a recording sheet as a recording body to a transfer nip part of a transfer region formed by an image carrier and a facing member that faces a surface of the image carrier, and then records on this recording sheet a toner image formed on the image carrier. The present invention particularly relates to an image forming apparatus, such as a copying machine, a facsimile device and a printer, which not only has a function for adjusting the pressure of a transfer nip formed by the contact between the image carrier and the facing member by appropriately adjusting a relative distance between the image carrier and the facing member, but also is capable of preventing the occurrence of image degradation, such as displacement of the position of an image, which is caused by load fluctuations of the image carrier generated when the recording sheet enters and is removed from the transfer nip.
2. Description of the Related Art
This type of image forming apparatus can not only adjust the pressure of the transfer nip formed by the contact between the image carrier and the facing member by adjusting the relative distance therebetween, but also alleviate load fluctuations of the image carrier caused when the recording sheet enters the transfer nip, the recording sheet formed from cardboard as a recording body. As distance adjusting means for adjusting the relative distance between the image carrier and the facing member, generally the one having a configuration for pushing back the facing member to a certain level against a biasing force, while biasing the facing member toward the image carrier by using biasing means.
For example, the distance adjusting means of the image forming apparatus described in Japanese Patent Application Laid-open No. H10-83124 has such configuration. Specifically, a transfer roller of this image forming apparatus that functions as the facing means has a cylindrical roller part and a shaft member protrudes from each end face of the roller part so as to rotate integrally with the roller part. The protruding parts of the shaft member are provided with eccentric cams so that the shaft member can spin. A motor for spinning the eccentric cams on a circumferential surface of the shaft member is directly connected to the eccentric cams. In the eccentric cams that are spun on the circumferential surface of the shaft member by the motor, cam surfaces are caused to abut with an end part of a photosensitive drum in a shaft line direction within a range of predetermined rotation angles. The transfer roller that is biased toward the photosensitive drum is moved against the biasing force away from the photosensitive drum by means of this abutment, whereby the inter-shaft distance between the photosensitive drum and the transfer roller.
However, in this conventional image forming apparatus, a drive transmission mechanism has to be provided in the vicinity of each end of a roller shaft line direction, in order to drive the eccentric cam spinning on the shaft member on the one end side of the transfer roller in the shaft line direction and the eccentric cam spinning on the shaft member on the other end side. Such layout restriction causes significant downsizing of the apparatus.
Note in this conventional image forming apparatus that the eccentric cams spinning on the shaft member of the transfer roller are caused to abut with the photosensitive drum. The same problem occurs in a configuration where the eccentric cams spinning on the shaft member of the photosensitive drum are caused to abut with the transfer roller to move the transfer roller away from the photosensitive drum. The same problem occurs also in a configuration where a belt member for endlessly moving at least either the photosensitive drum and the facing member while wrapping a rotatable support rotating body, the photosensitive drum and the facing member being brought into contact with each other to form the transfer nip.
When, on the other hand, a tip end of the recording body enters the transfer nip formed by the image carrier and a contact member, or when a rear end of the recording body is removed from the transfer nip, instantaneous load fluctuations occur in the image carrier, disturbing the surface movement speed of the image carrier. As a result, so-called shock jitter occurs, which is image degradation caused by displacement of the position of an image. Such image degradation occurs easily if the basis weight of the recording body is as great as that of cardboard.
For example, Japanese Patent Application Laid-open No. 2007-334292 describes an image forming apparatus that has gap forming means for separating an image carrier and a contact member to form a gap therebetween at predetermined timing immediately before a tip end of a recording body enters the position of a transfer nip where the image carrier and the contact member contact with each other, or immediately before a rear end of the recording body is removed from the position of the transfer nip. In this apparatus, formation of the gap between the image carrier and the contact member at the timing immediately before the entry of the tip end of the recording body or immediately before the separation of the rear end of the recording body can reduce the load fluctuations that occur in the image carrier at the time of the entry or separation of the recording medium.
Furthermore, in an apparatus that performs transfer using a transfer electric field formed by the transfer nip described above, the transfer electric field is formed by starting to apply transfer voltage to the image carrier or the contact member before the entry of the tip end of the recording medium into the transfer nip, by using transfer voltage application means. When the transfer voltage application means starts applying the transfer voltage after the tip end of the recording body enters the transfer nip, the output of the transfer voltage application means does not increase before transfer of an image tip end part starts, and consequently a sufficient transfer electric might not be obtained at the image tip end part. This is because, in this case, image degradation called a tip end part transfer failure occurs.
However, when the transfer voltage is applied before the tip end of the recording body enters the transfer nip in order to prevent the occurrence of the tip end part transfer failure, this conventional apparatus causes adverse effects in which leak discharge occurs at the gap between the image carrier and the contact member that is formed immediately before the entry into the transfer nip, damaging the image carrier.
Technologies relating to the present invention are (also) disclosed in, e.g., Japanese Patent Application Laid-open No. H4-242276, Japanese Patent No. 3,822,266, and Japanese Patent No. 3,911,941.